In laser processing machines, it is preferable to know an actual position (X-Y position) of the laser beam within the laser processing nozzle and a focus position (Z position) of the laser beam relative to a workpiece surface as accurately as possible or to orientate the laser beam, that is to say, to position the laser beam at a desired position relative to the nozzle opening and/or relative to the workpiece surface. To that end, a contact point of the laser beam on an edge of the nozzle opening or the edge of another, suitably positioned body can be established.
The desired position of the laser beam relative to the laser processing nozzle is generally at the center of the nozzle opening. The orientation is normally carried out manually in the prior art. To that end, an adhesive strip is adhesively bonded over the nozzle opening and a small hole is burnt into the adhesive strip at low laser power. The deviation of the beam position from the nozzle center is determined with the naked eye and a magnifying glass. Corresponding positioning units are used to correct the beam position and have adjustment screws extending at right-angles relative to the nozzle axis, by means of which the position of the laser beam can be changed in an X direction and Y direction in an X-Y plane of the nozzle opening. That operation is imprecise, time-intensive and unsuitable for automated process operations owing to the manual actions which are necessary.
U.S. Pat. No. 5,574,225 and JP-04295711A disclose a device and a method for establishing the position of a pulsed laser beam by means of two or more acoustic sensors which have an exact position relative to a plate-like optical element that is arranged in the beam path. The pulsed laser beam triggers an acoustic wave in the optical element owing to the photoacoustic effect, in which the acoustic wave is detected by the acoustic sensors. The phase difference of the acoustic wave signal, which is introduced into the acoustic sensors in a time-staggered manner, is used as a measure for the displacement of the laser beam relative to the optical axis which extends through the center of the optical element. The devices described in U.S. Pat. No. 5,574,225 and JP-04295711A require very precise positioning of the acoustic sensors for accurate orientation of the beam.
Various methods are known for establishing the focus position of the laser beam, that is to say, the position of the beam focus relative to the workpiece surface. JP-63108985A discloses a method using the photoacoustic effect. The beam diameter on a workpiece varies during the movement of the laser processing head of the laser processing machine in the propagation direction of the beam. In principle, a small beam diameter on the workpiece results in great intensity of the acoustic signal produced, and vice versa. If the focal point is in the plane of the workpiece, the diameter of the laser spot on the material surface is at a minimum and the intensity of the photoacoustic signal is at a maximum. If maximum intensity is detected when the acoustic signal is evaluated, the focal point is in the plane of the workpiece and the focus position is established as a result.